MICROSCOPIC EXAMINATION OF PREPARATIONS 187 



is especially true of the different varieties of bacillus dysen- 

 terise, and of the group of typhoid and paratyphoid organ- 

 isms. Further study will, no doubt, reveal variations in 

 other pathogenic bacteria, which varieties are today regarded 

 as distinct species. 



MICROSCOPIC EXAMINATION OF PREPARATIONS. 



The Different Parts of the Microscope. — Before describing 

 the method of examining preparations microscopically, a 

 few definitions of the terms used in connection with the 

 microscope may not be out of place. (The different parts 

 of the microscope referred to below are indicated by letters 

 in Fig. 34.) 



The ocular or eye-piece (a) is the lens at which the eye is 

 placed when looking through the instrument. It serves to 

 magnify the image projected through the objective. 



The objective (b) is the lens which is at the distal end of 

 the barrel of the instrument, and which serves to magnify 

 the object to be examined. 



The stage (c) is the shelf or platform of the microscope on 

 which the object to be examined rests. 



The diaphragms are the perforated stops that fit in the 

 centre of the stage. They vary in size, so that different 

 amounts of light may be admitted to the object by usipg 

 diaphragms with larger or smaller openings. 



The "iris" diaphragm (d) opens and closes like the iris 

 of the eye. It is so arranged that its opening for admission 

 of light can be increased or diminished by moving a small 

 lever in one or another direction. 



The reflector (e) is the mirror placed beneath the stage, 

 which serves to illuminate the object to be examined. 



